In my aforesaid copending application there is described a process and apparatus for recycling used asphalt-aggregate compositions. That apparatus, of which the present invention constitutes an improvement, includes an elongated cylindrical drum having a plurality of heating tubes extending substantially along the length of the drum interior. Used asphalt-aggregate compositions are recycled by introducing them into one end of the drum and rotating the drum while applying heat to the interior of the tubes so that the composition becomes heated gradually by contact with the heated tube surfaces. The description of that apparatus and the method of its use is incorporated herein by reference.
The heating of asphalt containing compositions or mixtures without subjecting the material to flame or hot gases, as set forth in my prior application, is highly advantageous over processes and apparatus such as described in U.S. Pat. No. 3,423,222 in which the materials are directly exposed to flame emitted from a burner or similar heat source. Not only does such exposure to direct flame or hot gases cause burning and deterioration of asphalt but the resulting smoke and vapors drawn into the atmosphere are obviously a pollution hazard.
Previous methods of heating a composition utilizing tumbling action within a cylinder such as described in the aforesaid patent are quite inefficient since the only portion of the material being heated is that exposed on the surface of the mass as it cascades in the rotating cylinder. Such a disadvantage is eliminated utilizing the heating pipes thus not only avoiding contamination and burning of the asphalt, since there is no exposure to flame, but greatly increases the efficiency of heating since a significant amount of material is in contact with the hot pipes. Moreover, complete mixing of the composition is greatly facilitated because the pipes act to deflect the material in different directions as the drum or cylinder rotates thereby increasing the mixing effect during the tumbling and heating operation with the result of improving the composition's homogeneity.
However, even with the improved apparatus in which heating uniformity and efficiency are much improved as is mixing, it is found that when introducing the materials having a significant amount of moisture thereon such as used asphalt-aggregate composition or aggregate which has become wetted, there is much water vapor or moisture given off during the heating process. The presence of significant amounts of moisture may interfere with asphalt coating characteristics especially as condensation occurs on cooler aggregate and interior drum walls. Thus, the moisture is preferably removed from the interior of the drum so as not to interfere with the asphalt coating of the aggregate as well as to facilitate composition temperatures in the desired ranges of about 275.degree.-325.degree. F with expenditure of unnecessary fuel. As such temperatures are approached, especially in the composition at the hotter end of the drum chamber, hydrocarbon volatiles and smoke are given off from the hot asphalt and which materials if vented directly into the atmosphere are obviously undesirable atmospheric pollutants. It is to the elimination of this problem as well as to further improve heating and mixing efficiency in producing asphalt-aggregate compositions that the present invention is directed.